2 Replies

High availability simply refers to the fact that the server system is in some way tolerant to failure.

That can be helped, in general, through hardware redundancy. For example, if a machine has redundant power supplies, then if one fails the machine will keep running. You can go a long way using hardware redundancy.

On top of that, you then have application redundancy, which usually refers to the ability for an application running on multiple hardware installations to respond to clients in a consistent manner from any of those hardware installations. That way, if the hardware does totally fail, or the O/S dies on a particular machine, another machine can carry on.

SQL Server caters for application redundancy in three four ways:

Clustering

Mirroring

Replication

Log Shipping (thanks Jack!)

Hope that clears it up a bit, but I'm sure some other people will have some good things to say too!

I'd add log shipping to the the HA solution although with replication and log shipping the failover is manually while with Clustering and Mirroring (depending on the way it is setup) is supposed to be automatic.